Electrocoagulation (EC) is a water treatment process that has been adapted to remove a number of contaminants from wastewaters, including fine suspended solids, toxic organic matter, silica, and heavy metals (Emamjomeh and Sivakumar, 2009; Mollah et. al, 2001). EC uses an electrochemical cell to generate coagulating agents in the wastewater by electrochemical reactions, generally resulting in flocculation of contaminated particles which separate out by settling or flotation. Conventionally, coagulation and flocculation are performed by the addition of chemicals including metal salts or polymeric flocculants. The electric field used in EC generally enhances the flocculation process by setting the charged colloidal particles in motion resulting in the coagulation of even very fine suspended solids (Harif and Adin, 2007). In addition, EC may be adapted to simultaneously remove heavy metals by precipitation as the pH increases during the process. Furthermore, hydrogen gas bubbles generated by reduction of water at the cathode (2 H2O+2e−→H2+2 OH−) may lead to floatation of the flocculated particles to the water surface thus providing better separation of contaminants.
The process makes use of an electrochemical cell with two electrode plates, an iron or aluminum anode and a cathode also usually made of iron or aluminum (Liu et al., 2010). Other anode materials have also been reported in literature such as copper (Ali et al., 2013), zinc (Ali et al., 2013; Fajardo et al., 2015), and magnesium (Vasudevan et al., 2010). The wastewater stream is the electrolytic solution.
When current is passed, the anodic material is oxidized to its respective cations which dissolve into the water stream. These metal cations will hydrolyze to form metal hydroxy cations or metal hydroxide precipitates which are excellent coagulating agents. The cationic coagulants generated will neutralize the negative surface charge present on the colloidal matter such as suspended solids or emulsified oil droplets. Hence the electrostatic repulsion among the colloidal particles which hinder separation by settling is eliminated leading to predominantly Van der Waals attraction among the particles which causes coagulation (Liu et al., 2010). The neutral metal precipitates can also coagulate colloidal particles and other contaminants by adsorption of onto the precipitates. The larger aggregates then flocculate and separate out by settling or flotation.